Wear Your Heart on Your Sleeve

First appears in Shakespeare's Othello (1604), spoken by the villain Iago, who is actually describing the opposite of sincerity: he plans to fake openness as a manipulative disguise, saying "I will wear my heart upon my sleeve / For daws to peck at," meaning he'll make himself look vulnerable purely to deceive people. The image itself likely predates the play. In medieval jousting tournaments, a knight would sometimes tie a token from a chosen lady to his sleeve before competing, a visible, wearable declaration of devotion that Shakespeare's phrase probably draws on.